DEVELOPEMENT OF BATEACIIIA. 



621 



segments. In b, the cervical constriction begins to define the 

 head from the trunk : the complete coalescence of the myelonal 

 tracts obliterates the linear trace of the median furrow, and the 

 neurapophysial rudiments border the myelon. The embryo and 

 its supporting yolk-mass are separated from the chorion by a clear 

 fluid ; and in the above-figured stages of devehjpement the ciliated 

 epithelium begins to act upon the fluid in the direction indicated 

 by the arrows, proceeding backward and downward along the 

 sides : the currents are strongest on the hasmal arches, from which 

 the branchlaj are about to be developed. In the mass of em- 

 bryonal cells Ijetwecn the cephalic enlargement of the embryo 

 and the yolk, the heart, fig. 4.31, r, is formed, which becomes 

 hollow, and pulsates before the red blood appears ; when the 

 communication with the system of vessels is established, the 

 heart propels blood, at first pale and with spherical corpuscles, 

 in channels formed by liquefaction of cells in the blastema of 

 the second ha3mal arch ; and these primary vascular arches esta- 

 blish the communication with the longitudinal aortic trunk simi- 

 larly formed along the under part of the notoehord. The blood 

 returns by venous channels along the yolk, now progressively 

 becoming inclosed by the lateral intestinal plates, and the simple 

 clrcnlatlon is complete. 



From the substance around the vascular arches are formed as 

 many branchial arches, as subordinate developcmeuts from the 

 second primary hfcmal or visceral arch ; 

 and from the branchial arches are 

 budded the succession of vascular loops 

 and coextended ciliated integument, 

 constitutlnjr the outer irills on each 

 side of the batrachian larva, fig. 430. 

 In the magnified portion of the gill, c, 

 the arrows indicate the direction of the 

 ciliary currents. Soon after the ap- 

 pearance of the heart, and of the arches 

 which encompass the primitive bucco- 

 branchial cavity, a pericardium, lined 

 with epithelial cells, is formed around 

 the heart. Between the cephalic hcemal arches interspaces are 

 opened, communicating with the bucco-branchial cavity, and from 

 one of these the budding gills begin to protrude. 



The growth of the neurovertebral axis is chiefly lengthwise, 

 and, as it proceeds, its two extremities lift themselves above the 

 level of the rest of the germinal basis ; the shorter and more 



430 



Larva oi Frog ; A, nat. 



ccxxxTirr. 



